1. Field of the Invention
This invention relates to novel bis-dithiobenzilnickel complexes which are used as near-infrared absorbing materials and to novel benzoin derivatives which are useful as intermediates of the complexes.
2. Description of the Prior Art
Bis-dithiobenzilnickel complexes such as bis-dithiobenzilnickel, bis-(4-dimethylamino)-dithiobenzilnickel, and bis-(4-diethylamino)-dithiobenzilnickel have strong absorption in the near-infrared region of 700-1,300 nm, and thus are known to be used as near-infrared absorbing materials or organic materials for optical disks, as reported, for example, in Senryo To Yakuhin (Dyes and Chemicals), 35 5 (1990) [4-25], Japanese Patent Laid-open Publication 63-227597, and Japanese Patent Laid-open Publication 64-61432.
On the other hand, an optical recording medium comprising a near-infrared absorbing material and a thermal recording material is disclosed in Japanese Patent Laid-open Publication 2-120082. However, these complexes show dark green colors. Since optical recording materials based on paper using these complexes are required to have high brightness, complexes of paler in color have been in demand. Furthermore, since the maximum absorptions of these complexes differ between in a solid state and in solution, especially when coated on paper, the maximum absorptions are shifted from those of the solution. Therefore, when these complexes are applied to an optical recording material using a semiconductor laser (wavelength: 830 nm), it is necessary to use one which has a maximum absorption in the vicinity of 830 nm in a solid state. However, the known bis-dithiobenzilnickel complexes do not have suitable maximum absorptions near 830 nm, for example, bis-dithiobenzilnickel has its maximum absorption in 850-870 nm, bis-(4-dimethylamino)-dithiobenzilnickel has a maximum absorption in 1,060-1,070 nm, and bis-(4-diethylamino)-dithiobenzilnickel has a maximum absorption in 1,100-1,120 nm.